The present invention relates to a cruise control apparatus for a vehicle which serves to automatically maintain the speed of the vehicle at a constant target level by properly controlling the opening degree of a throttle valve under the action of a motor or the like.
Japanese Patent Laid-Open No. 62-218239 discloses this type of cruise control apparatus in which a drive mechanism is provided for controllably driving a throttle actuator, which is connected with a throttle valve in an intake manifold of an internal combustion engine independently of an accelerator pedal, on the basis of a speed signal representative of the vehicle speed in combination with control signals such as a cruise control set signal, a cruise control reset signal, a cruise control resume signal, etc., to control the speed of a vehicle.
With this type of conventional cruise control apparatus, however, particularly in cases where it is used in combination with a fuel injection system, when the vehicle enters a downhill during travelling at a constant speed under cruise control, as illustrated at (a) in FIG. 3, the throttle actuator controls the throttle valve to move it to a fully closed position (i.e., an idling position) in order to suppress the vehicle speed, as illustrated by a solid line at (c) in FIG. 3. On this occasion, if the fuel injection system operates to cut off the fuel supply, the vehicle speed abruptly decreases. Accordingly, the throttle valve is then controlled to open so that it moves from the idling position and stops the fuel cut-off. Accordingly, the amount of intake air sucked into the engine increases suddenly, so there develops an abrupt change in the output torque of the engine, as shown by a solid line at (d) in FIG. 3, thus giving rise to a great shock. Subsequently when the vehicle speed increases to exceed a target value, similar events recur. As a result, the vehicle speed is subject to hunting in a cycle of 5 to 6 seconds which is undesirable and very unpleasant to the passengers in the vehicle.